Caustic solutions provide an effective means to remove H2S from acid-gas streams, but co-absorption of CO2 is often a problem because CO2 absorbs more strongly than H2S. Absorption of CO2 wastes the caustic solution and also may cause fouling due to the precipitation of carbonate solids (e.g., sodium carbonate decahydrate).
One solution has been to use a co-current contactor with very low residence times to minimize co-absorption of CO2. Since H2S can be absorbed much more quickly than CO2 at high pH levels, selective H2S removal is accomplished by limiting the contact time between the gas and the liquid in a static mixer. See, e.g., U.S. Pat. No. 4,824,645 to Jones, et al.
Maximizing the absorption of H2S and minimizing the absorption of CO2 is typically controlled by varying the rate addition of fresh caustic, solution circulating rate and column characteristics such as packing height or number of trays. See, e.g., GasTIPS, “Simulator Provides Guidance for Increasing CO2 Slip in Gas Treating Applications”, Weiland, Ralph H., et al., volume 8, no. 1, Winter 2002: 21-28. However, the low residence times used can limit the efficiency of the scrubber.
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Thus, there is still a need for improved caustic scrubbers that reduce co-absorption of CO2 by the caustic solution without requiring low residence times.